1. Field of the Invention
The present invention relates to digital printing apparatus and methods, and more particularly to a multipress, lithographic printing system with components that can be linked together in a modular fashion.
2. Description of the Related Art
In offset lithography, a printable image is present on a printing member as a pattern of ink-accepting (oleophilic) and ink-repellent (oleophobic) surface areas. Once applied to these areas, ink can be efficiently transferred to a recording medium in the imagewise pattern with substantial fidelity. Dry printing systems utilize printing members whose ink-repellent portions are sufficiently phobic to ink as to permit its direct application. Ink applied uniformly to the printing member is transferred to the recording medium only in the imagewise pattern. Typically, the printing member first makes contact with a compliant intermediate surface called a blanket cylinder which, in turn, applies the image to the paper or other recording medium. In typical sheet-fed press systems, the recording medium is clamped to an impression cylinder via grippers, which brings it into contact with the blanket cylinder.
In a wet lithographic system, the non-image areas are hydrophilic, and the necessary ink-repellency is provided by an initial application of a dampening (or "fountain") solution to the plate prior to inking. The ink-abhesive fountain solution prevents ink from adhering to the non-image areas, but does not affect the oleophilic character of the image areas.
If a press is to print in more than one color, a separate printing member corresponding to each color is required. The original image is transformed into a series of imagewise patterns, or "separations," that each reflect the contribution of the corresponding printable color. The positions of the printing members are coordinated so that the color components printed by the different members will be in register on the printed copies. Each printing member ordinarily is mounted on (or integral with) a "plate" cylinder, and the set of cylinders associated with a particular color on a press is usually referred to as a printing station.
Traditionally, the plates for offset presses have been produced photographically. However, to circumvent the cumbersome photographic development, plate-mounting and plate-registration operations entailed by this process, practitioners have developed electronic alternatives that store the imagewise pattern in digital form and impress the pattern directly onto the plate. Plate-imaging devices amenable to computer control include various forms of lasers. U.S. Pat. Nos. 5,351,617 and 5,385,092 (the entire disclosures of which are hereby incorporated by reference) disclose an ablative recording system that uses low-power laser discharges to remove, in an imagewise pattern, one or more layers of a lithographic printing blank, thereby creating a ready-to-ink printing member without the need for photographic development. In accordance with those systems, laser output is guided to the surface of the printing blank and focused onto that surface (or, desirably, onto the layer most susceptible to laser ablation, which will generally lie beneath the surface layer).
In most conventional presses, the printing stations are arranged in a straight or "in-line" configuration. Each such station typically includes an impression cylinder, a blanket cylinder, a plate cylinder and the necessary ink (and, in wet systems, dampening) assemblies. The recording material is transferred among the print stations sequentially, each station applying a different ink color to the material to produce a composite multi-color image. Another configuration, described in U.S. Pat. No. 4,936,211 (the entire disclosure of which is hereby incorporated by reference), relies on a central impression cylinder that carries a sheet of recording material past each print station, eliminating the need for mechanical transfer of the medium to each print station. With either type of press, the recording medium can be supplied to the print stations in the form of cut sheets or a continuous "web" of material.
Difficulties can arise in the manufacturing or use of in-line presses when the number of printing stations becomes large, particularly in the case of cut-sheet recording media. Passage from each printing station to the next involves a separate "handoff" of the page, requiring delicate mechanical feeding movements and, consequently, providing opportunity for slippage or malfunction. Even a small degree of slippage during one handoff may result in large overall distortions or misregistrations, because the error is propagated and amplified as the page travels through the remainder of the printing path.
Central impression designs reduce printing errors arising from paper handoff by minimizing the number of times a sheet is actually transferred. The sheet may, for example, be withdrawn from a bin and affixed to the central impression cylinder in a single operation, and stripped from the cylinder only after traversing all printing stations. In this way, misregistration errors are substantially reduced, since the opportunity for paper slippage between stations is removed. Furthermore, any errors resulting from initial paper handling are not amplified, since the orientation of the paper with respect to the printing stations remains essentially fixed.
Unfortunately, the number of printing stations that can be usefully employed in a central-impression design is limited by the circumference of the impression cylinder. It is mechanically unwieldy and economically prohibitive to employ cylinders capable of accommodating more than just a few such stations. Furthermore, central-impression designs do not lend themselves to "perfecting" operations whereby the facewise orientation of a sheet is reversed so that ink can be applied to the other side.